To prevent undesirable loads for which the wind turbine is not designed, its components must be manufactured according to maximum dimensional and mass tolerances, among others.
One of the most critical components in this regard are the rotor blades since deviations in their nominal weight and static moment relative to the rotor axis can introduce high loads and vibrations in the wind turbine. These must not only be manufactured individually according to a maximum weight tolerance, the tolerance usually given as a percentage of the nominal weight, but some conditions need to be verified in the relationship between the values taken by parameters of the two blades for two-bladed rotors, or the three blades in the case of three-bladed rotors, forming the rotor.
These parameters are:                maximum difference in weight between the blades;        static moment of rotor imbalance with respect to the rotor axis.        
Corrections often have to be made on the weight of the blades to achieve that not only individually, but also the pairs and trios of blades of a wind turbine rotor fall within the established tolerances. To this end balancing chambers are commonly arranged inside the blades, designed to accommodate the amount of material necessary to achieve this depending on the magnitude of the above parameters.
Not performing adequate correction of dispersions in mass between the blades to achieve a proper balance of the rotors and that the resultant static moment from all of them over the rotor shaft is zero, may have important implications in terms of loads and vibrations in wind turbines on which they are mounted. Sometimes these implications involve costly on-site actions, once the blades have been installed on the wind turbine, and to mitigate them it is necessary to implement methods to balance the rotors on site such as that described in the U.S. Pat. No. 5,140,856 A or the document “Reducing vibration by balancing rotor blades”. Erneuerbare Energien August 2009. Prüftechnik.
Factory balancing methods are also implemented. Specifically, the measuring and/or characterization step of weight and/or static moment of the blades is performed. Some methods for characterizing the static moment of the blades are described in patents EP2233904B1 and U.S. Pat. No. 4,078,422.
A valid approximation to consider that a rotor is balanced is when the vectorial sum of static moments of the blades relative to the rotor axis is zero. It is generally considered that a rotor meets the specifications if the resultant static imbalance is below a threshold value taken into account when calculating the loads and design of the wind turbine.
If these specifications are not met by the blades as manufactured, the rotors must be balanced, this being achieved generally by placing masses at certain points of the blades a posteriori.
However, even applying the above methods and considering that the resultant static imbalance is below the threshold value, it is possible that the blades may present or be under certain intrinsic or extrinsic conditions, which greatly increase stresses in the wind turbine, leading to considerable decreases in the fatigue life of its components.
The consequence of this is that wind turbines whose rotors have a resultant static imbalance which is admissible at the discretion of the threshold value, undergo however increased vibrations caused by the mass distribution of the blades, which results in longitudinal accelerations which are damaging to the wind turbine nacelle.
The known wind turbine control methods implement protection algorithms to prevent such vibration levels from exceeding a certain threshold and they proceed to stop the operation of the wind turbine when they detect dangerous levels.
On the other hand, certain weather events associated with low temperatures can cause ice to adhere in an imbalanced manner between the blades of the wind turbine, contributing to aggravate the existing imbalance. In situations in which stresses are increased in the wind turbine due to vibrations, it is possible that the protection algorithms may stop the wind turbine much sooner than they would if there was not already a noxious imbalance.
The wind turbine rotor balancing method of the present invention overcomes all the above drawbacks allowing not only to reduce vibrations and loads on the wind turbine but also the number of stops associated with rotor imbalances, which will help to reduce production losses associated with stops after detecting excessive vibration, while increasing the life of the wind turbine.